Unit 3 Pt 2: Genetics Grade 11 AP Bio (Heredity)

Heredity

The passing of genetic traits from parent to offspring

Traits

Characteristics that can be passed on

Genetics

The branch of biology that deals with the principles of variation

3 Early Theories About Heredit

Pangenesis

An entire human being was inside the sperm

Blending hypothesis

Pangenesis (proposed by Aristotle)

Suggested that sperm & egg contained tiny particles from all body parts

Males and females formed genes in every organ

Genes moved: blood -> genitals -> children

Blending Hypothesis

Traits from the parents were irreversibly blended into the offspring

Red flower + white flower = pink flower

Around the 1800s

Gregor Mendel

Studied garden peas - had clear cut characteristics that are easy to follow

Each trait only had 2 possible variations

Mendel's Two Innovations:

1. Pure Lines - plants that only produce the same variety as the parent plant

2. Counted his results and kept statistical notes

How Mendel made true breeding lines

Cut off anthers

Put pollen from anthers onto the one he wanted pollinated

Cut small squares and string and tied them around each flower - prevent animal pollination

Records which pod came from which plant

Pollinated a tall pea plant with a short pea plant

Particulate Inheritance (Mendel)

Combined traits passed from parents to offspring through genes

Traits are passed on as genes, do not blend

3 Principles of Mendelian Genetics

1. Law of Dominance

2. Law of Segregation (Mendel wanted it to be the first law)

3. Law of Independent Assortment

1. Law of Dominance

Each sperm and egg cell has a factor(n), when you put them together you get a certain trait - we call these genes today

Two new terms - dominant & recessive alleles

Phenotype/genotype expression is based on genes

Allele

Different code, located at a gene

Represented by any letter (except X or Y)

Dominant

The allele the expresses itself at the expense of an alternate allele

Recessive

The allele whose expression is suppressed in the presence of a dominant allele

Phenotype

The appearance of the trait in an organism (P)

Genotype

The specific allelic combination for a certain gene or set of genes

Monohybrid crosses

When only one trait is monitored at a time

2. Law of Segregation

Traits are determined by pairs of alleles that separate in gamete formation

Each gamete receives only one allele

These alleles randomly unite at fertilization

Inherited traits are determined by pairs of genes

Occurs in metaphase I

3. Law of Independent Assortment

Alleles for one trait segregate independently from alleles of another trait during gamete formation

Each allele combination is equally likely to occur

Mendelian Monohybrid Cross Ratio

3:1

Mendelian Dihybrid Cross Ratio

9:3:3:1

Test Cross

Used to determine whether an individual is homozygous or heterozygous for a dominant trait

How to Test Cross

1. Individual with an unknown genotype is crossed with a homozygous recessive individual

2. Phenotype of the offspring will reveal the unknown genotype

- All offspring display dominant phenotype = parent must be homozygous for trait

- Half of the offspring show the dominant phenotype, other half is recessive = parent must be heterozygous for trait

Incomplete dominance

When neither allele is completely dominant over the other allele

A new heterozygous phenotype appears as a blend of the dominant and recessive phenotypes

Incomplete Dominance Examples

Four o' clock flowers/Snapdragons

Anadalusian Chickens

Codominance

When two equally dominant alleles are expressed at the same time

Heterozygous phenotypes will have both phenotypes visible

Codominance Examples

Red & white roses

Shorthorn Cattle (roan)

Checkered chickens

Sickle Cell Anemia

Sickle Cell Anemia (codominance)

A misshapen red blood cell that can't transport oxygen efficiently

Creates blood clots

Inefficient at delivering O2

Heterozygotes who carry the trait - some normal some sickle cells

Resistant to malaria - parasite cannot fit into the cell

HbA - normal

HbS - anemia

HbSHbS - cannot live

Multiple Alleles

When there are more than 2 possible alleles for a gene

ABO Blood Types

Rhesus Factor

Rabbit Fur

ABO Blood Types (Multiple Alleles)

A & B are co-dominant

O is recessive

Represented by IA, IB, and i

Makes antigens for its blood type and antibodies for the opposite blood type

Rhesus factor - Rh+/- (Multiple Alleles)

Common to primates

An antigen(protein) that exists on the surface of red blood cells

Makes antigens for its blood type and antibodies for the opposite blood type

(+) + (+) = +

(+) + (-) = +

(-) + (-) = -

Rabbit Fur Colors (Multiple Alleles)

Determined by 4 alleles

C = full color (wild type), dominant

cch = chinchilla, partial pigmentation, dominant to ch and c

ch = himalayan - color in certain parts of the body

Black color is only expressed in cold regions of the body - vice versa

Influenced by environmental factors

c = albino, no color expressed, recessive to all other alleles

Pleiotropy

When a gene has multiple phenotypic effects

Thomas Hunt Morgan

Studied fruit flies in the basement of Columbia University

Originally believed proteins coded for life instead of DNA

Created experiment to prove it was proteins since fruit flies have few chromosomes

Why Morgan Chose Fruit Flies (5) (Sex-linked)

1. Fast life cycle

- 40 - 50 days from birth to death

2. High fecundity

- Female fruit fly can lay eggs within the first 24-48 hours of life

- Can lay up to 500 eggs in a lifetime

3. 8 pairs of chromosomes

- 2n=8

4. Visible Mutations

- Distinct observable traits

- Eye color, wing shape

5. Cheap

Wild type

Dominant allele

Sex determination

The sex of an individual is determined by the sex chromosomes that are contributed to the zygote

Egg can donate x, sperm can donate x or y - determines sex

Nomenclature (Sex-linked)

Symbols for genes are not used in the punnett square

Sex chromosome is used with symbols for dominant or recessive alleles as a superscript

DO NOT USE X OR Y FOR OTHER PUNNETT SQUARES

Sex-linked Inheritance

When the gene is located on the X/Y chromosome

Most known sex-linked traits are X-linked

- X chromosomes is large than Y chromosome

- Chromosomes do not line up - 78 genes on Y chromosome

Gene on the X chromosome is x-linked

- Can assume is x-linked unless told

Gene on the Y chromosome is y-linked

Inheritance of X-linked diseases/disorders

Males are at greater risk of inheriting disorders since they only inherit one X from their mom

Females can be carriers, need both X chromosomes to be affected

Muscular dystrophy (X-linked)

An inherited disorder that involves rapidly worsening muscle weakness

Rare in females - not usually passed on since person dies young

Results in:

- Loss of movement

- Diaphragm and intercostal muscles cannot move

- Death usually occurs by age 25 from lung disorders

Hemophilia

Inability to clot blood

Causes heavy bleeding and large bruises

Hemophilia in Males

Hemophilia allele is located on X chromosome - males do not have 2 copies

Allele's effects are not masked

Hemophilia in Females

Inherits only one allele from mother that is carrier

Can also inherit one recessive from both parents - people can live longer

Extra/missing chromosomes

Extra or missing copy can be lethal

Sex chromosomes are not affected - females can have 2 X chromosomes

Barr bodies

Inactivate X chromosome in female mammals

One chromosome in each cell becomes almost completely inactivated during embryonic

X chromosome is only unrolled enough to duplicate and split during mitosis

Calico Cat (X-linked, Trisomy)

- Male with white, orange, and black has Klinefelter's (Trisomy XXY)

- Allele for black and orange is carried on X chromosome

- In females: one allele on each X chromosome

- Patches are formed when barr bodies roll up the other chromosome

- Certain areas will grow colored/uncolored

Humans & Mammals (Sex Determination)

X-Y System

Birds & Some Reptiles (Sex Determination)

ZZ & ZW system, mom is ZW

Sea Worms (Sex Determination)

If larvae lands on sea floor, becomes female, if lands on female becomes male

Insects (Sex Determination)

Haplodiploid system

Fertilized (2n) are female

Non fertilized (n) are male

Reptiles (Sex Determination)

Some species are all female and hatch female clones of themselves

Turtles (Sex Determination)

Warm temperature = female

Cool temperature = male

Fish (Sex Determination)

Born as males, mature into females

Have a hierarchical system

Pedigree

Analyses the inheritance of alleles in a family

Autosomal Dominant Inheritance

A single copy of the allele is sufficient to express trait

Not located on a sex chromosome(autosomal)

Progeria (Autosomal Dominant)

Person ages very rapidly, die before reproducing

Does not create a pedigree

Huntington's Disease (Autosomal Dominant)

CNS breaks down around the age of 30

Does create a pedigree since individuals lives to reproductive maturity

Conditions for Autosomal Dominant Inheritance

1. Every affected individual has at least one affected parent

2. Males & females should be equally often affected

3. An affected person has at least a 50% chance of transmitting the dominant allele to each offspring

Autosomal Recessive Inheritance

2 recessive alleles result in a trait being expressed

Albinism (Autosomal Recessive)

Loss of pigment in the hair, skin, and eyes

Tay Sachs (Autosomal Recessive)

Build up of fatty deposits in the brain, eventually fatal

Conditions for Autosomal Recessive Inheritance

1. An affected person does not have to have affected parents, parents could be carriers

2. Both sexes are affected equally, can "skip" generations if all are carriers

3. 2 affected parents will have affected children 100% of the time

X-linked Dominant Inheritance

When a single dominant allele on the x chromosome can lead to a trait

Very uncommon

Vitamin D Resistant Rickets (X-linked Dominant)

Leads to bone deformities, bowed legs

Conditions for X-linked Dominant Inheritance

1. Twice as many females as males - 2:1

2. Half of the kids of an affected female will have it - sex is not considered- 1:1

3. All daughters of an affected male will be affected but not the sons - 1:1

X-linked Recessive Inheritance

When the recessive allele is on the x chromosome

Conditions for X-linked Recessive Inheritance

1. Males are more likely to get it than females - 8:1 ratio

2. Affected males will transmit the allele to all daughters, but not to sons

3. Homozygous recessive females can arise only from matings in which the father is affected and the mother is a carrier

Polygenic Traits

The control of the expression of a trait by several genes

Continuous Variation

Phenotypes vary gradually from one extreme to another

Linked Genes

When alleles that are very close to each other on the same chromosome do not assort independently

The genes are too close together for crossing over to be able to separate them

Does not produce 9:3:3:1 ratio

Chromosome mapping

Tracking how the frequency is based on the allele's proximity in the chromosome

Epistasis

2 separate genes, one affects the other

Produces less than 4 phenotypes (usually 3)

Recessive Epistasis

When the recessive allele of one gene masks another

At the locus - when the gene is located

9:3:4 ratio

Dominant Epistasis

Dominant allele overrides expression of other alleles at the other gene

12:3:1 ratio

Statistical significance

The likelihood that a relationship between two or more variables is caused by something other than chance

Provides evidence concerning the plausibility of the null hypothesis

X2 (Chi-square)

Determines if 2 categorical variables from the same sample population are independent of each other or related

Has a bell curve shape

Degrees of freedom (df)

x number of options - 1

How many choices can be made?

Null hypothesis (Ho)

There is no substantial statistical difference between the observed and expected values

The results that do exist are totally random and occurred by chance alone

Alternative Hypothesis (H1)

There is a substantial difference between the observed and expected values

The results/differences are not random and are influenced by other factors

Table of Critical Values

The table that contains the values of the X2 curve

Significance Levels

Probability values in the top row of the table

0.10(10%), 0.05(5%), 0.01(1%)